When drilling for oil and gas, it is desirable to maintain maximum control over the drilling operation, even when the drilling operation may be several kilometers below the surface. Steerable drill bits can be used for directional drilling and are often used when drilling complex borehole trajectories that require accurate control of the path of the drill bit during the drilling operation.
Directional drilling is complicated because the steerable drill bit must operate in harsh borehole conditions. The steering mechanism is typically disposed near the drill bit, and the desired real-time directional control of the steering mechanism is remotely controlled from the surface. Regardless of its depth within the borehole, the steering mechanism must maintain the desired path and direction and must also maintain practical drilling speeds. Finally, the steering mechanism must reliably operate under exceptional heat, pressure, and vibration conditions that will typically be encountered during the drilling operation.
Many types of steering mechanism are used in the industry. A common type of steering mechanism has a motor disposed in a housing with a longitudinal axis that is offset or displaced from the axis of the borehole. The motor can be of a variety of types including electric and hydraulic. Hydraulic motors that operate using the circulating drilling fluid are commonly known as a “mud” motors.
The laterally offset motor housing, commonly referred to as a bent housing or “bent sub”, provides lateral displacement that can be used to change the trajectory of the borehole. By rotating the drill bit with the motor and simultaneously rotating the motor housing with the drillstring, the orientation of the housing offset continuously changes, and the path of the advancing borehole is maintained substantially parallel to the axis of the drillstring. By only rotating the drill bit with the motor without rotating the drillstring, the path of the borehole is deviated from the axis of the non-rotating drillstring in the direction of the offset on the bent housing.
Another steering mechanism is a rotary steerable tool that allows the drill bit to be moved in any chosen direction. In this way, the direction (and degree) of curvature of the borehole can be determined during the drilling operation, and can be chosen based on the measured drilling conditions at a particular borehole depth. Rotary steerable tools can be configured as point-the-bit or push-the-bit to steer drilling.
Typically, the rotary steerable tool uses a reference to the tool's position while drilling so the rotary steerable tool can steer the advancing borehole in the correct direction. Because the rotary steerable tool rotates in the advancing borehole and experiences a number of disturbances in the process, the rotational speed of the tool can vary significantly over the course of several and even a single rotation. For example, Stick-Slip is one type of variation that can occur in the rotational speed of the steering apparatus. Stick-Slip can produce inaccuracies that cause significant difficulties in controlling the trajectory of the borehole. Therefore, accurate sensing capabilities of the rotary steerable tool in high resolution is beneficial to system performance, allowing the rotary steerable tool to better compensate for downhole dynamics.
Although several rotary steerable tools available in the industry may be effective, they may still suffer from inaccurate operation due to the dynamic conditions that can occur downhole while advancing a borehole.